MAX8785ETI+;MAX8785AETI+;MAX8785ETI+T;MAX8785AETI+T;中文规格书,Datasheet资料

General Description

Liquid-crystal display (LCD) enclosures and cold-cathode fluorescent lamps (CCF L) used in notebook computer and portable electronic displays are becoming increa-singly narrow, generating the need for a low-profile CCF L power supply. Recent advances in piezoelectric transformers (PZTs) have made it possible to develop smaller, more efficient, and safer backlight inverters for portable displays. Piezoelectric transformers have shown better performance than magnetic transformers with respect to efficiency, EMI requirements, human safety, and form factor. The MAX8785A is a full-bridge CCF L controller for piezoelectric transformer-based backlight power supplies. The full-bridge topology pro-vides a high-spectral purity sinusoidal drive that helps the PZT convert electrical energy to mechanical and back to electrical energy efficiently.

The MAX8785A employs a feed-forward control architec-ture that provides excellent line-and-load regulation while maintaining relatively constant switching frequency.The MAX8785A provides protection against open-lamp,secondary short-circuit, lamp arcing, and secondary overvoltage with adjustable timeout periods.

The MAX8785A guarantees lamp striking by sweeping the switching frequency from high to low until the lamp is struck. The MAX8785A achieves 10:1 dimming range using a digital pulse-width modulation (DPWM) https://www.360docs.net/doc/da2897663.html,FL brightness can be set with an analog voltage on the CNTL pin or through an external signal at LSYNC.The maximum switching frequency and DPWM frequen-cy can be adjusted with external resistors.

The MAX8785A gate drivers can drive large power MOSF ETs typically used in high-power CCF L applica-tions. An internal 5.35V linear regulator powers the MOSFET drivers and most of the internal circuitry. The MAX8785A is available in a low-profile, 28-pin thin QFN package and operates over the -40°C to +85°C tempe-rature range.

Applications

Notebook LCDs LCD TVs and Monitors

Features

o Resonant, All n-Channel, Full-Bridge Topology o Wide Input-Voltage Range (4.5V to 28V)

o Frequency Sweeping Guarantees CCFL Striking o Feed-Forward Control Provides Excellent Line-Transient Response

o Programmable Maximum Switching Frequency o Analog or Digital DPWM

o Lamp-Out Detection with Adjustable Timeout o Fault Protection with Adjustable Timeout o Primary Current Limit with R DS(ON)Sensing o Strong Gate Drivers Support Large External MOSFETs

28-Pin Thin QFN Package

MAX8785A

Full-Bridge Controller for Piezoelectric Transformers

________________________________________________________________Maxim Integrated Products 1

Pin Configuration

Ordering Information

For pricing, delivery, and ordering information,please contact Maxim Direct at 1-888-629-4642,or visit Maxim’s website at https://www.360docs.net/doc/da2897663.html,.

+Denotes lead-free package.*EP = Exposed pad.

Simplified Operating Circuit appears at end of data sheet.

M A X 8785A

Full-Bridge Controller for Piezoelectric Transformers

ABSOLUTE MAXIMUM RATINGS

ELECTRICAL CHARACTERISTICS

(V BATT = 12V, V DD1= V DD2 = V SHDN = 5.35V, R RATE = 190k Ω,T A = 0°C to +85°C , unless otherwise noted. See Figure 1. Typical values Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability.

BATT, RATE, LX1, LX2 to GND .............................-0.3V to +30V BST1, BST2 to GND ...............................................-0.3V to +36V BST1 to V DD1, BST2 to V DD2..................................-0.3V to +30V BST1 to LX1..............................................................-0.3V to +6V BST2 to LX2..............................................................-0.3V to +6V V CC , V DD1, V DD2, CNTL, LSYNC, SHDN ,

FLT to GND...........................................................-0.3V to +6V GH1 to LX...............................................-0.3V to (VBST1 + 0.3V)GH2 to LX2.............................................-0.3V to (VBST2 + 0.3V)GL1 to GND..............................................-0.3V to (V DD1+ 0.3V)GL2 to GND..............................................-0.3V to (V DD2+ 0.3V)

COMP, HF, LF, PCOMP, SEL,

TFLT to GND............................................-0.3V to (V CC + 0.3V)IFB, VFB, OLF to GND.................................................-6V to +6V PGND to GND .......................................................-0.3V to +0.3V Continuous Power Dissipation (T A = +70°C)

28-Pin Thin QFN (derate 21.3mW/°C above +70°C)...1702.1mW Operating Temperature Range ...........................-40°C to +85°C Junction Temperature......................................................+150°C Storage Temperature Range.............................-65°C to +160°C Lead Temperature (soldering, 10s).................................+300°C

MAX8785A

Full-Bridge Controller for Piezoelectric Transformers

ELECTRICAL CHARACTERISTICS (continued)

(V BATT = 12V, V DD1= V DD2 = V SHDN = 5.35V, R RATE = 190k Ω,T A = 0°C to +85°C , unless otherwise noted. See Figure 1. Typical values

M A X 8785A

Full-Bridge Controller for Piezoelectric Transformers 4_______________________________________________________________________________________

ELECTRICAL CHARACTERISTICS (continued)

(V BATT = 12V, V DD1= V DD2 = V SHDN = 5.35V, R RATE = 190k Ω,T A = 0°C to +85°C , unless otherwise noted. See Figure 1. Typical values

ELECTRICAL CHARACTERISTICS

MAX8785A

Full-Bridge Controller for Piezoelectric Transformers

_______________________________________________________________________________________5

ELECTRICAL CHARACTERISTICS (continued)

M A X 8785A

Full-Bridge Controller for Piezoelectric Transformers 6_______________________________________________________________________________________

Typical Operating Characteristics

(Circuit of Figure 1. V IN = 12V, T A = +25°C, unless otherwise noted.)

LOW INPUT-VOLTAGE OPERATION

(V IN = 12V)

10μs/div

CH1: 10V/div CH2: 2V/div CH3: 1V/div

I FB V FB

LX MAX8785A toc01

V CNTL = 2.0V SEL = GND

HIGH INPUT-VOLTAGE OPERATION

(V IN = 24V)

10μs/div

CH1: 20V/div CH2: 2V/div CH3: 1V/div

I FB

V FB

MAX8785A toc02

V CNTL = 2.0V SEL = GND

LINE-TRANSIENT RESPONSE

40μs/div

CH2: 2V/div CH4: 10V/div

I FB

V IN

MAX8785A toc03

12V

24V

MINIMUM BRIGHTNESS OPERATION

AND STARTUP WAVEFORM

4ms/div

CH1: 2V/div CH2: 5V/div

CH3: 5V/div CH4: 2V/div

I FB

COMP

SHDN

V FB

MAX8785A toc04

LSYNC OPERATION

1ms/div

CH1: 2V/div CH3: 5V/div CH4: 10V/div

I FB

LSYNC

MAX8785A toc05

LAMP-OUT VOLTAGE LIMITING

AND TIMEOUT

200ms/div

CH1: 2V/div CH2: 2V/div

V FB

TFLT

MAX8785A toc06

SECONDARY SHORT-CIRCUIT PROTECTION AND TIMEOUT

200μs/div

CH1: 2V/div CH2: 1V/div

V FB

TFLT

MAX8785A toc07

400μs/div

MAXIMUM SWITCHING FREQUENCY

vs. R HF

R HF (k Ω)

M A X I M U M S W I T C H I N G F R E Q U E N C Y (k H z )

120

140

100

160

10080

20DPWM FREQUENCY vs. R LF

R LF (k Ω)

D P W M F R

E Q U E N C Y (H z )

200250

300

100

150

350

400

30020010035025015050

0M A X 8785A t o c 09

MAX8785A

Full-Bridge Controller for Piezoelectric Transformers

V CC LINE REGULATION

V IN (V)

V C C (V )16

5.3625.3605.3585.3565.3615.3595.3575.3535.3555.3545.352

M A X 8785A t o c 10

V CC LOAD REGULATION

I LOAD (mA)

V C C R E G U L A T I O N (%)

0.20

0.100.05-0.050.150-0.15-0.10-0.20

M A X 8785A t o c 11

V CC vs. TEMP

TEMPERATURE (°C)

V C C (V )

-15

-40

5.3705.360

5.3555.3455.3655.3505.3355.3405.330

M A X 8785A t o c 12

SWITCHING FREQUENCY vs. V IN

V IN (V)

f S W (k H z )

1820

57.0

56.856.6

56.4

56.2

56.0

M A X 8785A t o c 13

Typical Operating Characteristics (continued)

(Circuit of Figure 1. V IN = 12V, T A = +25°C, unless otherwise noted.)

_______________________________________________________________________________________7

M A X 8785A

Full-Bridge Controller for Piezoelectric Transformers 8_______________________________________________________________________________________

MAX8785A

Full-Bridge Controller for Piezoelectric Transformers

_______________________________________________________________________________________9

M A X 8785A

Figure 1. Typical Operating Circuit

Full-Bridge Controller for Piezoelectric Transformers 10______________________________________________________________________________________

Typical Operating Circuit

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